Satellite communication is widely used to provide communication in a variety of commercial and consumer arenas. For example, satellite communication is widely used for telephone communication, television transmission and reception as well as internet service.
Recently, government requirements have been made to provide a system of providing the location of telephone communications under an enhanced 911 system. Under this system as to land telephone lines the provider provides systems and software capable of determining the position from which the call is made in order that emergency services can be provided. Similarly enhanced 911 requirements are now being imposed which require the providers of cellular telephone service to provide a location from which the cellular telephone call has been made. Such systems rely on positioning systems to determine the location of the cellular phone.
Global Positioning Satellite (GPS) systems have been widely used since the 1980's to provide positioning information to a receiver for commercial utilization within the United States. Such global positioning systems are used elsewhere in the world and, for example, Russia operates a similar system (GLONASS) and the European Union is developing a third system “Galileo”.
GPS systems use a combination of orbiting satellites and land stations to communicate with a receiver to provide signals which may be utilized to determine the position of the receiver. Such global positioning systems are widely used for a variety of commercial purposes including providing vehicular travel location assistance, common surveying systems, a variety of agricultural and commercial applications, and/or military usage.
GPS receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS (or NAVSTAR) satellites. These satellites transmit, as part of their message, both satellite positioning data, as well as, data on clock timing, so called (EPHEMERIS data). Ephemeris data provides the location or positioning of a satellite at a particular time, as well as, the positions at later times for the satellite. The process of sending and acquiring GPS signals, processing ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, requiring at least 30 seconds for a given satellite to broadcast its complete ephemeris. In many cases, this lengthy process time is unacceptable, and furthermore, greatly limits battery life in microminiaturized portable applications.
For receivers that are small, widely used, and relatively inexpensive, for example cellular phones, the requirements under the enhanced 911 system include that the cellular phones provide global positioning system information. This lengthy processing time becomes extremely difficult when using this method of providing telephone transmission positions using global positioning systems in cellular telephone applications. The enhanced 911 requirement requires that the global positioning position for Network Based Solutions be within 100 meters for 67% of calls.
One limitation of current GPS receivers is that their operation is limited to situations in which multiple satellites are clearly in view without obstructions, and/or a good quality antenna is properly positioned to receive such signals. As such, they normally are unsuitable in portable body mount applications; in areas where there is significant foliage or building blockage; and in building applications.
There are two principle functions of global satellite receiving systems; 1) computation of pseudoranges to the various GPS satellite, and 2) computation of the position of the receiving platform using these pseudoranges and satellite timing and is extracted from the global positioning system satellite signal once it has acquired and tracked. Collecting this information normally takes a relatively long time (from 30 seconds to several minutes) and must be accomplished with a good received signal level in order to achieve low error rates.
Virtually all known GPS receivers utilize correlation methods to compute pseudoranges. These correlation methods are performed in real time often with hardware correlators. GPS signals contain high rate repetitive signals called pseudorandom or (PN) sequences. The codes available for PN applications are in the range of 0.1 millisecond. The code sequences belong to a family known as GOLD codes. Each GPS satellite broadcast has a signal with a unique GOLD code.
For signal received from a given GPS satellite, following a down conversion process to base band, correlation receivers multiply the received signal by a stored replica of appropriate GOLD code contained within its local memory and then integrates or low pass filters the product to obtain an indication of the presence of the signal. This process is termed a correlation operation. By subsequently adjusting the relative timing of this (acquisition). Once acquisition occurs, the process enters (tracking) phase in which the timing of the local reference is adjusted in small amounts to maintain a high correlation output. The correlation output during the tracking period may be viewed as the GPS signal with the pseudorandom code removed or in common terminology (despread). This signal is low band with band width commensurate with a 50 bit per second binary phase shift key signal data which is superimposed on the GPS waveform.
The GPS System is a satellite based navigation system made up of a network of 24 satellites that was developed by the US Department of Defense. It was originally intended primary for the US military to provide precise position, velocity, and time. Civilian use of GPS became available in 1980's. At the time, GPS became popular for use with civilian aircraft and surveying. Through the 1990's civil applications of GPS application grew at an astonishing rate. Today, GPS has found uses in many applications such as land transportation, civil aviation, maritime.
GPS navigation essentially involves solving for unknown receiver positions (Xr, Yr, Zr) and clock bias Ctbr, given measurements of pseudorange, ρ and correction terms for satellite clock bias and atmospheric effects.
The pseudorange is defined as the measured signal transit time multiplied by the speed of light in a vacuum. The measurement made by the GPS receiver is the apparent transit time of a signal from each satellite to the receiver. This is determined as the difference between the time the signal was received otherwise known as a pseudorange. In addition to the pseudorange, the satellite position (XS, YS, ZS) is required to solve the non linear systems of equations as shown as equation 1.1 below:ρ1=√{square root over ((Xr−Xs1)2+(Yr−Ys1)2+(Zr−Zs1)2)}{square root over ((Xr−Xs1)2+(Yr−Ys1)2+(Zr−Zs1)2)}{square root over ((Xr−Xs1)2+(Yr−Ys1)2+(Zr−Zs1)2)}+ctb,R ρ2=√{square root over ((Xr−Xs2)2+(Yr−Ys2)2+(Zr−Zs2)2)}{square root over ((Xr−Xs2)2+(Yr−Ys2)2+(Zr−Zs2)2)}{square root over ((Xr−Xs2)2+(Yr−Ys2)2+(Zr−Zs2)2)}+ctb,R ρn=√{square root over ((Xr−Xsn)2+(Yr−Ysn)2+(Zr−Zsn)2)}{square root over ((Xr−Xsn)2+(Yr−Ysn)2+(Zr−Zsn)2)}{square root over ((Xr−Xsn)2+(Yr−Ysn)2+(Zr−Zsn)2)}+ctb,R  (1.1)
The pseudorange in equation 1.1 represents a measurement that has already been corrected for the satellite clock bias in atmospheric effects. Where the super script refers to the satellite number, a minimum of four satellites are required to solve the receiver position and clock bias. Each satellite broadcasts a signal that has encoded ephemeris data about its own position an a lower-precision almanac which describes the orbits of all the satellites in the SPC constellation.
Conventional GPS receivers require time to acquire and lock onto the satellite signals. Once locked, the receiver downloads the almanac and ephemeris data from the signal. The process of downloading the ephemeris takes around 30 seconds. A high signal strength is necessary to decode the ephemeris at a sufficiently low bid error rate (BER) to allow calculation of the satellite position. Any severe attenuation of the signal can cause loss of lock and the signal will need to be re-acquisitioned.
Presently signal processing techniques exist, which allow the tracking of the GPS signal (and hence, the generation of the pseudorange measurements ρ above) at very low signal to noise ratios, C/No. The C/No is the ratio of the power in the receive signal [dB-W] to the power in the competing noise [dB-W/HZ].
Current research has shown that tracking is possible with C/No as low as 15 dB-HZ [38-40]. At such signal strength, the BER may be too low to reliably decode the data message and extract the satellite ephemeris, although the tracking will allow generation of pseudorange measurements. Also many weak signal tracking acquisition methods use a software receiver approach in which a segment of the ephemeris data is buffered into memory. The 30 seconds of data containing the ephemeris message may be too long to completely load into memory.
Providing satellite position information with a sufficient accuracy has been difficult with prior systems under conditions in which the C/No ratio is below the minimum threshold which is around 27 dB-HZ, for decoding the message. Therefore in the event that new ephemeris data can't be downloaded from the broadcast signal, the user may still compute their position within a desired accuracy for an extended period of time.
The motion of planets, as well as those of satellites, was first empirically modeled by Johannes Kepler in the early 17th century. From this model became Kepler's three laws of planetary motion, the first of which is that all planets move in an elliptical orbit having the sun about which they rotate as one focus [known as the law of orbits]. The second law is that a line joining any planet to the sun sweeps out equal areas in equal times [known as the law of areas]. The third law is that the square of the period of any planer about the sun is proportional to the cube of the planet's mean distance from the sun [known as the law of periods]. Mathematical models of orbiting of satellites are based on these well established principals.
The current GPS navigation message is comprised of quasi-Keplerian parameters and accounts for perturbations in several elements. These parameters can be used to compute each satellite position in the World Geodetic Survey of 1984 (WGS84) frame which is the Earth Centered Earth Fixed (ECEF) frame implemented by GPS. This element set is presented in Table 1.1 below.
TABLE 1.1Current Ephemeris Parameters in the GPS Navigation MessageParameterDescriptiontoeephemeris reference time{square root over (a)}square root of the semi-major axiseeccentricityi0inclination angle at the reference timeΩ0longitude of the ascending node at the beginning of the GPSωargument of perigeeM0mean anomaly at reference timeΔncorrection to the computed mean motionirate of change of inclination with timeΩrate of change of RAAN with timeCuc, Cusamplitudes of harmonic correction terms for thecomputed argument of latitudeCrc, Crsamplitudes of harmonic correction terms for thecomputed orbit radiusCic, Cisamplitudes of harmonic correction terms for thecomputed inclination angle
The harmonic terms account for perturbations at twice the orbit period and help compensate for some of the gravity effects due to low order Earth gravity harmonics. The fit interval for these elements is typically four hours, but they are updated every two hours to allow for a two hour overlap. The accuracy during this fit interval is, on the range, of around two to five meters. Outside of this four hours, the curve fit degrades quickly due to higher order Earth gravity harmonics, solar pressure, and third body perturbations.
The Russian counterpart to GPS is the Global Navigation Satellite Systems (GLONASS) which uses a Cartesian ephemeris format depicted on Table 1.2 below:
TABLE 1.2Current Ephemeris Parameter in the GLONASS Navigation MessageParameterDescriptionx, y, zsatellite position in PZ90 reference frame{dot over (x)}, {dot over (y)}, żsatellite velocity in PZ90 reference frame{umlaut over (x)}, ÿ, {umlaut over (z)}luni-solar acceleration in PZ90 reference frametereference time
The GLONASS system uses a similar ECEF frame as the WGS84 frame implemented by GPS. The GLONASS Parametry Zemli 1990 (PZ90) frame adopts a different set of defining parameters than WGS84 and therefore these two frames vary slightly. A conversion between these two frames is provided in Appendix A herein. The GLONASS standard fit interval is 30 minutes and achieves comparable accuracy as the GPS 4 hour fit ephemeris. Similar to the GPS ephemeris, the curve fit degrades quickly outside of this interval.
The U.S. Space Command uses the NORAD 2-line elements which are used to maintain and catalogue orbit data on all space projects. See Table 1.3 below:
TABLE 1.3NORAD 2-line elementsParameterDescription nmean motioneeccentricityiinclinationΩright ascension of the ascending nodeωargument of perigeeMmean anomaly       n    .    2first derivative of mean motion divided by 2       n    ¨    6second derivative of mean motion divided by 6 B*drag parameterUTCepoch
Table 1.3 outlines the elements of this set. The elements in this set are mean Keplerian elements which are obtained by removing the periodic variations from precise numerically integrated trajectories. These element sets provide similar accuracy to the GPS broadcasted almanac data which is on the order of kilometers.
This element set alone would not provide the accuracy needed for navigation and any equipment using this format to propagate the satellite orbits would need to know exactly how the perturbations were removed such as not to add additional error to the element fit
The GPS and GLONASS systems are limited in that they can't predict well outside their fit intervals and they have increasing fit errors for extended fit intervals. Keplerian based elements generally describe the perturbations to a satellite orbit better than quickly changing position and velocity vectors. A longer propagating element set would modify the current GPS ephemeris to account for higher order gravity harmonics and third body perturbations.
A popular solution for aiding E911 is referred to as assisted GPS. Assisted GPS is a method of assisting a cellular device with positioning capabilities to fit their location. This technology uses the base stations in the cellular network as well as a remote server. The cellular device will initiate a request for location information. The nearest cell tower will gather information from the remote server about which satellites should be in view. Satellite information and ranging measurements taken from the cellular network are sent to a remote server. The cellular device coordinates are either sent back to the handset or to the LBS server. This method allows most of the computation to be done at the remote server, which has higher processing capabilities and therefore can compute the handset position much quicker. This, however, requires a near real time communication link.
A need therefore exists to determine an element set that can propagate longer into the future with an accuracy applicable to meet the needs of enhanced 911 and location based services. Further, there is a need to provide a system that requires minimal data storage and may propagate with low cost on board processing. In addition, a system is required that would not need to implement a separate tracking system but utilize readily available tracking data. The present invention is adapted to address at least some of the aforementioned problems with the prior art